> My solution will be to get a Siemens C475IP or maybe the similar
> SNOM M3.

To be honest, that is exactly what I did (an S450IP in my case). The
advantage of a real VOIP-phone rather than an ATA is that the sent and
received signal never meet each other on a single line. The only echo will
be caused acoustically in the handset. So the chance of echo on the far end
is reduced quite a lot. I never had any complaints about echo anymore (I did
have some with my SPA3000, although not often).

Christian